A printed circuit board comprises a substrate of insulator material and a circuit pattern of conductive material commonly formed by photoimaging processes. For example, a fiberglass board to be printed with circuitry is plated with copper and laminated with a layer of photoresist material, a photosensitive polymer. The coated surface is then exposed to actinic radiation through a film with some areas which are transparent to the actinic radiation and some areas which are not, to define the desired circuit pattern. The pattern is developed as a relief image in the photoresist by solvent treatment, the exposed and unexposed portions of the photosensitive polymer having different solubilities. The printed substrate may be either a positive or negative image of the film depending on whether a positive or negative working photopolymer is used. Typically, the unexposed photoresist is removed and the underlying copper etched away to leave a pattern of conductive copper in the image of the transparent pattern on the film.
Photoimaging apparatus such as for the manufacture of printed circuits typically include glass plates and/or film coated with photographic emulsion, iron oxide or chromium films having a pattern of transparent and opaque areas with respect to actinic radiation. The aligned apparatus is exposed to actinic radiation which passes through the transparent areas of the image to photographically impose a pattern on the coated substrate, which is typically developed as a relief image by dissolution of the unexposed photosensitive material. In the alternative, the exposed coating material may be removed by dissolution, or either the exposed or unexposed areas may be chemically treated. The chemistry of photoimaging processes is well-known, and any one of the conventional routes is acceptable to arrive at a circuit pattern on the substrate.
A typical apparatus currently employed in the photoimaging processes described above generally comprises a pair of glass or transparent plastic frames defining an enclosed space which can be evacuated. Typically, a pair of struts, rods or pivots employing gas or hydraulics, are used to assist in opening, holding open, and closing the upper and lower frames. U.S. Pat. No. 5,072,257 to Stoesser, U.S. Pat. No. 4,600,298 to Emmert and U.S. Pat. No. 4,896,188 to Byers describe typical strut, rod and pivot devices. A pair of flexible photoimaging masters may be visually aligned and then manually secured individually to the frame members by such devices as vacuum, tape, clamps or spring clips. However, glass photoimaging assemblies are typically registered using cylindrical alignment pins fitted through holes drilled through corresponding areas of the glass plates.
A photopolymer coated substrate is then precisely registered between the frame members. Different thicknesses of substrates, especially thick or dual substrates, may affect the registration of the materials. The apparatus is subjected to vacuum and exposed to actinic radiation. When the substrate is removed from the apparatus to develop the circuit pattern, the top glass plate must be lifted, held in place by support means such as struts, rods or pivots to replace or check the substrate. The struts, rods or pivots hold the top frame in place, allowing the operator to easily change substrates or check registration. However, the constant lifting, holding in place and closing by the support means places a great deal of stress upon the lower glass frames since the load carried by the support means in holding the upper frame is transferred to the lower frame which causes the lower frame to deflect or bend. Typically, the most common type of repair work for photoimaging devices involves replacing or repairing the lower frame which has cracked or has broken due to stresses incurred by repeated openings and closings, Common breakages, especially for the widely used glass frames, involve hairline fractures down the center or skewed slightly off center of the lower frame.
In addition, should the thicknesses of substrates vary, the device must be manually shimmed up or down which may involve deconstructing a complicated hinge structure or even removing a hinge which may be bonded by an adhesive to the glass. This process is also difficult, time-consuming and inaccurate, with the glass exposed to high risk of breakage, and the hinge is compromised by repeated shimming, and lifting and closing. The present invention has no mechanical or wearing parts. Another option is to keep multiple photoimaging devices with each one having the ability to accommodate a different thickness of substrate. However, this is an expensive and impractical option, especially for smaller companies involved in the printed circuit industry. Others have attempted to create flexible hinges which can accommodate differing substrate thicknesses. U.S. Pat. No. 4,412,739 to Freiberg, U.S. Pat. No. 4,446,184 to Bowser, and U.S. Pat. No. 4,584,216 to Kenworthy describe typical flexible hinges.
The present invention provides an improved photoimaging apparatus which improves reliability by using a reinforcing hinge to counteract stresses caused in the frame members by support mechanisms such as struts and pivots, retains the inherent flexibility of the polymeric hinge material and provides a greater range of adjustability and serviceability of the photoimaging apparatus without compromising precise registration. The present invention, resistant to breakages common of other photoimaging assemblies, combines the inherent flexibility of a polymeric hinge with the ability for the user to easily adjust the photoimaging device with spacers of varying thicknesses to accommodate varying thicknesses of substrates and also allows the user to easily service and maintain the assembly which has no mechanical or wearing parts.